Broken String.prototype.startsWith polyfill

[apolakipso]

This overwrites String.prototype.startsWith and breaks any code that expects a modern implementation to be present (like Office web add-ins or wherever this file gets loaded).

Issue

This implementation of String.prototype.startsWith doesn't support the position parameter, while all modern browsers do (it's also in here now: https://tc39.es/ecma262/#sec-string.prototype.startswith) - with the egregious bonus of not even checking whether a polyfill is even required.

Proposed solution

• This polyfill should only be installed if String.prototype.startsWith does not exist.
• The polyfill should implement a modern version with support for the position parameter (which would be functionally equivalent for code that uses it without anyway)

referencesource/System.Web.Extensions/Script/String.js

Line 19 in 51cf785

String.prototype.startsWith = function(prefix) {

NB: I'm referring to this version of the file in the wild, please let me know if this is the wrong source repo
https://ajax.aspnetcdn.com/ajax/3.5/MicrosoftAjax.js